The invention relates to a method of rapid determination of the concentrations of the total sulphur dioxide and the lignosulphonates present in sulphite pulping process liquors.
The object of the invention was to provide a method for obtaining at short intervals measuring data on the concentration of the active "cooking" chemical for the sulphite cooking process, i.e. the compounds formed in the cooking liquor by the sulphur dioxide, and on the concentration of the lignosulphonate resulting from the pulping reactions for the control of the sulphite pulping process.
The invention is characterized in that the compounds which are formed in the liquor by the sulphur dioxide are separated from the lignosulphonates by means of an artificially effected ion exclusion phenomenon and that the concentration measurements are carried out from the liquor flow after the separation on the basis of the absorption of UV-radiation at a wave length of 280 nm. No mentions of a measuring method according to the present invention have been found in the literature.
In the digester house the cooking acid which is pumped into the sulphite pulping digester is usually analysed according to a certain standard by means of a laboratory titration method in order to determine the sulphur dioxide concentration. After the cooking has been started, no subsequent information will be obtained on the active cooking chemical. On account of variation in digester chip filling, wood density and moisture content as well as variation in the cooking process, the amount of lignin-dissolving sulphur dioxide compounds per amount of absolutely dry wood, however, varies from one charge to an other, which results in variation in the quality of the final product, which variation cannot be observed by means of the conventional measuring devices. Modern means for controlling cooking initial data, a mathematical model and a computer do not help solving these problems so that the need of devices following the actual cooking reactions and their proceeding is obvious. Attempts have been made to determine the concentration of the active cooking chemical during the cooking. In the lecture "On line cooking liquor analyzer--a means for effective control of sulfite digester", 1982 International Sulfite Pulping Conference, Toronto Oct. 20-22, 1982, TAPPI Proceedings, p. 285-292, Byland et al. disclose a method for controlling sulphite pulping, said method essentially comprising a sulphite cooking liquor analyzer operating according to the principle of titration. Said analyzer is realized in such a way that a titration method known per se has been automated, the concentration of the active cooking chemical being calculated at the equivalent point by means of a computer. Automating a titration method, however, requires several complicated and accurate feeding and dosing steps, which makes it liable to disturbances as well as expensive.